This invention relates to the comb filter art used in the television art. Specifically, a comb filter is provided which is relatively free of uncancelled signal components.
In the NTSC television system, video signals are developed which contain the necessary picture information for reconstruction of an image. The picture information is comprised of 525 consecutive raster lines. Each line has a chrominance and luminance signal associated therewith for identifying the color and brightness of the line. The lines which comprise the transmitted picture are produced at a frequency of F.sub.h which is typically 15.73426 kHz. The luminance and chrominance information therefore has a 15.73426 kHz repetition rate.
The frequency components of the luminance and chrominance signals are therefore centered at multiples of F.sub.h. The chrominance information is frequency translated into a chroma signal by modulating a subcarrier having a frequency corresponding to an odd multiple of 1/2 the line frequency. In the NTSC broadcast system the subcarrier is at a frequency of 3.579545 mHz.
Once the frequency translation has occurred, the resulting chroma signal is combined with the luminance signal to form the composite video signal. The sidebands of the subcarrier containing the chrominance signal are interleaved with the luminance signal because of the prior frequency translation.
In a television receiver the chroma and luminance signals are separated by known means. One technique, referred to as comb filtering by those skilled in the art, delays the video signal by 1/F.sub.h seconds. The delayed video signal is thereafter combined with an undelayed video signal. The delayed signal corresponds to the video signal of a previously scanned line and the undelayed signal corresponds to the video signal of a presently scanned line.
The chroma signal undergoes a phase reversal of 180.degree. between consecutive lines due to the frequency translation. Therefore, by combining the video signal of a first scanned line with the video signal of a subsequently scanned line, the chroma components will be cancelled because of the 180.degree. phase difference between consecutive lines of video signals leaving only a luminance signal.
The above technique works well where the chroma signal of a first line is correlated in both phase and amplitude with the chroma signal of a subsequent line. When the chroma signals of two succeeding lines are not so correlated, total cancellation does not occur and the luminance signal produced contains an objectionable interfering chroma signal. The uncorrelated chroma signals are produced when a transition in color occurs between successive lines. The different colors of the consecutive lines produce the uncorrelated chroma signals which results in incomplete cancellation.